The IEEE 802.11ac standard specifies downlink (DL) Multi-User Multiple-Input Multiple-Output (MU-MIMO) transmissions, which enable a wireless network device, such as a wireless access point or a mesh node, to simultaneously transmit data to multiple client devices. Because spatial streams intended for different client devices are transmitted simultaneously, MU-MIMO transmissions are implemented with the digital signal processing (DSP)-based beamforming technique in order to manage inter-user interference. The implementation of MU-MIMO including beamforming needs to be efficient and accurate enough to focus specific spatial streams toward intended client devices while generating nulls toward the other clients that are part of the same MU-MIMO burst. In particular, to sufficiently reduce inter-user interference, very accurate cannel state information between the antennas of the wireless network device and the antennas of client devices is required. However, even if channel state information is obtained very frequently to account for fast changing channel states, the accuracy of channel state information is still limited by measurement, quantization, estimation, and other errors.
Moreover, as stated above, the need for very accurate channel state information in the operation of MU-MIMO calls for performance of sounding and feedback processes with a very high frequency. For example, in typical implementations the interval between two sounding and feedback cycles may be approximately 20 milliseconds (ms). The overhead associated with sounding and feedback processes at such a high frequency quickly becomes prohibitive as the number of active MU-MIMO client devices increases. For example, a significant impact on the overall throughput of the system can be expected with just more than a few active MU-MIMO client devices.